1. Field of the Invention
The present invention relates to an apparatus and system for cooling molten core material resulting from the meltdown of a nuclear reactor. More specifically, the present invention is a system designed to prevent the "China Syndrome" in the case of a nuclear accident and proposes a passive system which optimally utilizes heat pipes to provide cooling and solidification of the molten core so that core boiling and containment breach do not occur.
2. Description of the Prior Art
Following the release of the "Rasmussen Report" (WASH-1400, August 1974), a great deal of attention has been focused on the risks of serious nuclear reactor accidents by not only the members of the nuclear industry and the appropriate governmental licensing bodies but also by interested and concerned members of the public at large. The Rasmussen Report concludes that the risks of such an accident are dominated by those potential accidents which lead to the melting of the reactor core. One possible consequence of a core melt has been popularly dubbed "The China Syndrome" in which it is prophesied that the molten core will fall to and melt through the bottom of the reactor vessel as well as the underlying containment structure. If such extreme results were to occur, the containment would be breached and large amounts of radioactive material could be released to the environment either by way of the atmosphere or by way of the ground water at the site.
In response to the possibility, albeit remote, of such dire consequences, various core catcher concepts have been developed in order that the molten core be prevented from breaching the containment building. One of the earliest core catcher concepts appears in U.S. Pat. No. 3,607,630 issued to John M. West et al on Sept. 21, 1971 and consists of a horizontal flat steel plate underlaid by cooling pipes through which water could flow convectively from a storage tank to a discharge pipe. Other artificially cooled core catcher concepts have subsequently appeared in U.S. Pat. Nos. 3,702,802 issued to G. Jansen Jr. on Nov. 14, 1972; U.S. Pat. No. 3,935,063 issued to Thomas L. Dunckel on Jan. 27, 1976; and U.S. Pat. No. 4,036,688 issued to Martin P. Golden et al on July 19, 1977.
U.S. Pat. No. 3,702,802 is directed to a ceramic oxide eutectic core catcher with a relatively low melting point intended to dissolve the molten core to disperse and dilute the core material in a manner which permits the molten material to be more readily cooled and solidified. U.S. Pat. No. 3,935,063 discloses an arrangement of heat pipes which is intended not only to cool the core itself before it has melted but also to externally cool the vessel to prevent the melted core from melting through the bottom of the vessel. It is not made clear in that patent, however, whether such containment on the interior of the vessel results in a non-critical arrangement or how the top of the vessel is protected against melting. It is also not made clear how the heat pipe arrangement can be expected to work since the proposed apparatus requires the liquid phase of the working fluid of the heat pipe to be raised many feet; a feat which is not ordinarily practical. Furthermore, in all known arrangements in which a container is externally cooled, the system operates in a critical mode since it confines a material which may have a bulk temperature thousands of degrees higher than the melting point of the container. Miscalculations of the system's performance could result in melt-through. Distribution of the molten mass over a large surface area to distribute the heat load and to reduce it to a non-critical configuration is therefore essential. Finally it is not made clear in that patent how access is obtained to the reactor vessel exterior for testing during in service inspections.
U.S. Pat. No. 4,036,688 proposes a complicated core catcher which collects and distributes the molten core after it melts through the reactor vessel. The apparatus is basically a sacrificial bed system including a conical funnel, a core debris receptical including a spherical dome, a spherically layered bed of primarily magnesia bricks, and a zig-zag piping cooling system.
While each of these patents discloses a concept for catching and cooling a molten core, they are each subject to the same or similar difficulties. One difficulty shared by U.S. Pat. Nos. 3,607,630; 3,702,802; and 4,036,688 is that the proposed cooling systems consist of lengthy, sinuously winding or otherwise fluidly connected piping systems which pass adjacent to but under the surface to be cooled. In such systems, the breach of the piping at only one point would render the entire cooling system inoperative. If a pipe break were to occur in a position under the molten core, a circumstance which is not at all unlikely in view of the tremendous heat and weight of a dropped 160 ton core, a dangerous steam explosion might occur releasing large quantities of radioactive material for dispersal throughout the containment, which would, in all likelihood, prevent entry of humans into the containment building foreverafter. In any event, large amounts of steam would rapidly be generated which would have to be condensed in order to prevent unacceptable pressure buildup within the containment. In addition, such extensive arrays of interconnected pipeing are difficult to construct in a manner that would satisfactorily meet the seizmic requirements placed on nuclear power plants.
A second difficulty shared by all of the above mentioned patents is that their cooling systems transfer heat from the molten core to a heat exchanger positioned elsewhere within the containment building since it is not wise to permit such a system to penetrate the containment envelope for fear of pumping radioactive fluid to the exterior of the containment. These arrangements require the existance of a cooling capacity within the containment which greatly exceeds the cooling requirement of any accident less severe than a core meltdown. Thus, the prior art systems pose the burdensome requirement of providing an immense cooling capacity within the containment even though it would hopefully never be called upon to operate. Such cooling equipment would have to be maintained in an operational state for the life of the plant.
Other patents exist which propose the use of in-core heat pipes for the purpose of extracting heat from the core of the reactor during normal power operation. Such patents are exemplified by U.S. Pat. No. 3,378,449 issued to J. J. Roberts et al on Apr. 16, 1968; U.S. Pat. No. 3,403,075 issued on Sept. 24, 1968 to P. Fiebelmann; U.S. Pat. No. 3,509,386 issued to A. W. Byrd issued on Apr. 28, 1970; and U.S. Pat. No. 3,668,070 issued to P. Fiebelmann on June 6, 1972. These patents do not, however, address the problem of catching and solidifying a core that has melted due to a serious mishap.